It is frequently necessary, e.g. in the case of audio tape equipment or record players, to very exactly regulate the rpm of a drive motor and maintain it constant over long time periods. This object is typically achieved by using a negative-feedback rpm-regulating system in which a feedback signal indicative of actual motor rpm is compared with a reference signal indicative of desired motor rpm, the reference signal being derived from a quartz-crystal oscillator of well-defined frequency. In such systems, when the desired rpm has been established, thereafter the energization of the motor is controlled by regulation of the phase shift of the motor relative to the reference frequency; the rpm per se does not fluctuate, except to the extent incidental to such phase regulation.
Various circuits are known which can implement this type of regulation. Frequently, use is made of integrated circuits specially developed for such purpose. However, these are not always capable of being used to realize all the system criteria of a particular application; they generally do not provide sufficient degrees of freedom in the selectability of various parameters, such as for example the frequency which the pulse train from a feedback tachometer will have at a particular desired rpm. The type of regulation in question can, alternatively, be implemented by means of a suitably programmed microprocessor. Usually, however, this is economically feasible only when, for example, the equipment being powered by the motor includes such a microprocessor for other purposes, and where the unused capacity of the microprocessor is sufficient to accommodate and implement the motor-regulation program. Thus, it can happen that, if the special integrated circuits are not suitable for a particular application, and if a microprocessor is not conveniently present, one may be obliged to revert to more elementary rpm regulation without the benefits of phase regulation.